Hydroxymethyl aryl monocarboxylic acids such as p-hydroxymethylbenzoic acid, m-hydroxymethylbenzoic acid, and 6-hydroxymethyl-2-naphthoic acid are known in the art. Of particular interest has been p-hydroxymethylbenzoic acid (p-HMBA), which has been synthesized for use as a monomer to make the corresponding homopolymer, poly(p-methylenebenzoate) and its corresponding ester, methyl p-hydroxymethylbenzoate (mep-HMB). The acid is believed to have been prepared first as early as 1872 by free-radical bromination of p-toluic acid to p-bromomethyltoluic acid, hydrolysis with aqueous barium hydroxide, and subsequent purification by recrystallization from water.
A process has now been found for the hydrogenation of arylene dicarboxylic acids wherein one carboxylic acid group is hydrogenated to a hydroxymethyl group using a rhenium catalyst and yet is selective enough to prevent hydrogenolysis of the hydroxymethyl group to the methyl group and reduction of the second carboxylic acid to a hydroxymethyl group or to a methyl group. More specifically, a process has been found for the preparation of p-hydroxymethylbenzoic acid by the reduction of terephthalic acid in the presence of a rhenium catalyst under relatively mild conditions wherein conversion of terephthalic acid is within the range of from about 20 to 30(wt)% and selectivity to p-hydroxymethylbenzoic acid is within the range of from about 80 to 90%. Yields are accordingly within the range of from about 18 to about 27(wt)% of terephthalic acid feed. The invented process can be used also to prepare m-hydroxymethylbenzoic acid and 6-hydroxymethyl-2-naphthoic acid.
Rhenium oxide catalysts have been known as excellent catalysts for the liquid-phase reduction of a variety of organic substrates. Broadbent, et al., J. Organic Chem., 28, 2345 (1963) teaches rhenium VI oxide is a very efficient catalyst for the hydrogenation of carboxylic acids and carboxamides. Broadbent reported that benzoic acid, ethylbenzoate, benzaldehyde, and m-nitrobenzaldehyde were reduced to the corresponding aromatic carbinols instead of suffering hydrogenolysis to the toluenes which result from most catalytic hydrogenations. Broadbent, et al., ibid., 24, 1847 (1959) teaches that the catalytic substrate reduction activity of rhenium catalysts on aliphatic compounds differs for rhenium catalysts, also concurrently reduced, using different solvents. Broadbent teaches, op. cit., that rhenium heptoxide is reduced in situ as catalyst and reduces maleic acid with no solvent to 91% succinic acid and 9% 1,4-butanediol; that succinic acid with no solvent is reduced to 94% 1,4-butanediol and 6% n-butyl alcohol; but that succinic acid in p-dioxane is reduced to 61% butyrolactone, 33% 1,4-butanediol and 6% polyesters. Trivedi, et al., JAOCS, 17, January 1981, teaches that addition of ruthenium on carbon (Ru/C) to a rhenium heptoxide (Re.sub.2 O.sub.7) catalyst gave a synergistic effect in reduction of an aliphatic monocarboxylic acid to a primary alcohol at a temperature of 170.degree. C. and 2500 psi in a 1,4 dioxane solvent but at higher temperatures and lower pressures (230.degree. C. and 500 psi) hydrocarbons were the major or exclusive products. J. E. Carnahan, et al., JACS, 77, 3766, July 20, 1955, teaches that in rutheniumcatalyzed hydrogenation of adipic acid in water, hexamethylene glycol resulted at temperatures of 150.degree. to 175.degree. C. and pressures of 520-700 atmospheres. However, none of the above publications teach, disclose or suggest that a rhenium catalyst can be used to hydrogenate arylene dicarboxylic acids to hydroxymethyl aryl monocarboxylic acids wherein one carboxylic acid group is hydrogenated to a hydroxymethyl group and the other acid group is not reduced.
Unexpectedly it has been found that a rhenium oxide catalyst has catalytic activity in aqueous solvent to selectively reduce arylene dicarboxylic acids to the hydroxymethyl aryl monocarboxylic acids. Only one acid group of the arylene dicarboxylic acid is reduced to a hydroxymethyl moiety.
Accordingly, it is an object of this invention to prepare hydroxymethyl aryl monocarboxylic acids from arylene dicarboxylic acids by hydrogenation using a rhenium catalyst.
It is a further object of this invention to prepare p- and m-hydroxymethylbenzoic acids by reduction of terephthalic acid and isophthalic acid in the presence of a rhenium catalyst and an aqueous solvent.
It is a further object of this invention to provide a process for the preparation of p- and m-hydroxymethylbenzoic acids wherein terephthalic acid and isophthalic acid are directly reduced to p- and m-hydroxymethylbenzoic acids instead of to p- or m-xylene or dihydroxymethylbenzenes. Other objects will appear hereinafter.